The present invention relates to pumps and more specifically to blood pumps, ventricular assist devices, and artificial hearts.
The natural heart functions in a fashion similar to a positive displacement pump. Each of the two pumping chambers in the natural heart has two check valves (an inlet and an outlet valve). The walls of the natural heart are made of contractile muscle that provide the power to pump the blood. Each pumping cycle consists of a filling or diastolic phase of the pumping cycle and an ejection or systolic phase of the pumping cycle. During the filling phase, the muscle fibers making up the walls of the heart relax allowing the chamber they surround to fill with blood. During the ejection phase of the cycle the muscle making up the walls of the heart contracts ejecting a portion of the blood from the chamber. The check valves assure one-way flow.
Mechanical blood pumps have been developed for use as artificial hearts to replace or assist the natural heart. Present blood pumps which are available to assist or replace the heart fall into two general categories. One category uses a rotary impeller and includes centrifugal pumps and axial flow pumps. The other category is pulsatile pumps, the diaphragm type pump being the most common. Blood pumps may also be classified as internal (intracorporeal) or external (extracorporeal) to the body.
Diaphragm pumps are favored as they provide desirable pulsative flow and are reliable owing to their simplicity. Prior art diaphragm pumps comprise a housing, a flexible but not extensible diaphragm that divides the interior of the housing into two chambers, namely a pumping chamber and a driving chamber. Diaphragms are conventionally fabricated from polyurethane, a flexible but not elastic material. The pumping chamber portion of the housing has an inlet and an outlet, each of which is equipped with a one-way flow check valve. The diaphragm is driven into and out of the pumping chamber mechanically, pneumatically or hydraulically. Mechanical drives typically include a pusher plate on the drive side of the diaphragm connected to a cam, solenoid or other device to impart reciprocal motion to the pusher plate and diaphragm. Alternatively, a drive fluid, either liquid or gas, may be used to reciprocally drive the diaphragm into and out of the pumping chamber.
One of the problems associated with available mechanical blood pumps is the formation of blood clots (thrombosis) in the pump. To address this problem, the interior surfaces of the diaphragm and housing walls that define the pumping chamber are typically designed to have a very smooth surface, in an effort to retard clotting. Other attempts to reduce clotting have involved provision of a rough texture on the interior surfaces of the pumping chamber to encourage endothelial cells, normally lining the heart and blood vessels, to grow over the surfaces eventually providing a smooth surface. Both of these methods work to some degree, but clotting in the device, with clots breaking off and entering the circulatory system, remains a problem.
Another problem relates to the flow of blood through the pump. Significant turbulence occurs in the chamber during the pumping cycle. There is little that can be done to control the characteristics of blood flow through the pumping chamber. There are areas of high velocity and other areas of slow flow. These slow flow areas also contribute to clotting. Turbulence leads to energy loss and inefficiency of the pump. Excessive turbulence may also damage the blood cells.
An additional problem is rupture of the diaphragm. If the diaphragm is driven pneumatically or hydraulically, should a tear or rupture of the diaphragm occur, the driving fluid may be pumped into the bloodstream, causing a harmful and potentially fatal embolism. Even if the pump is mechanically driven, a diaphragm rupture can result in air entering the bloodstream causing an embolism.
The foregoing are long standing problems in the art that have defied solution. There is, therefore, a need in the field for an improved blood pump and ventricular assist device.
It is an object of the invention to provide a blood pump that reduces the incidence of blood clotting.
It is another object of the invention to provide a blood pump with improved flow characteristics, particularly, to reduce or eliminate stagnant and low velocity flow areas within the pumping chamber to reduce blood clot formation, and to minimize areas of high turbulence to avoid damage to blood cells.
It is also an object of the invention to prevent intrusion of foreign matter into the bloodstream, and especially to prevent embolisms of the driving fluid or other fluids as a result of a pump failure.
In attainment of these and other objects and advantages of the invention, a pump is provided that has an elastic, extensible or stretchable bladder that expands in the filling phase and contracts in the ejection phase of the pumping cycle. The pump is particularly well suited for pumping blood, as in a ventricular assist device or a total artificial heart. However, the pump of the invention will find applications in other industries and non-medical fields for pumping fluids other than blood. The summary and following detailed description is in reference to, but is not limited to, blood pumping applications.
In a most basic embodiment, the blood pump comprises a bladder, the interior surface area and volume of which is changeable, i.e., it stretches and expands during the filling phase, and elastically contracts to its normal relaxed size during the ejection phase. The bladder has a fluid inlet and a fluid outlet. A device, such as a vacuum pump, compressor, solenoid or cam, alternately expands and contracts the interior surface area and volume of the bladder. A majority of the interior surface area of the bladder expands and contracts a significant amount (more than a few percent) in each cycle. One or more check valves or other means for causing substantially one-way fluid flow through the bladder are also provided.
Looking at the normal heart, there is very little tendency for blood clots to form in the heart when it is working normally. When it is working normally, the muscle which comprises the walls of the heart contracts with each ejection changing the surface area of the lining of the heart. After a patient has sustained a myocardial infarction (heart attack) a portion of the heart muscle comprising the wall of the heart has become necrotic (dead) and a scar has formed in that area. Because that area of the heart is now a scar, rather than muscle, and can no longer contract, it does not change the surface area of the lining of the heart in this localized area. It has been discovered that in this localized area of the natural heart (the area that does not contract due to a previous heart attack) there is a significant tendency for blood clots to form. This suggests that the change in surface area of the lining of the heart, with each pump cycle, is important in preventing clot formation on the lining of the heart. In a similar fashion, the changing of the surface area of the bladder of the invention as it stretches and contracts with each pumping cycle will decrease or eliminate clot formation on the surface of the bladder.
In a preferred embodiment, the blood pump of the invention comprises a housing, an extensible bladder in the housing, and a void volume or space between the housing and the bladder adapted to be occupied by a driving fluid. The bladder has an inlet and an outlet. At least one check valve is provided at the bladder inlet and/or outlet to provide one-way flow through the bladder. A vacuum source, compressor or other means is provided for altering the pressure of the driving fluid to alternately expand and contract the interior surface area and volume of the bladder. In the preferred embodiment, the driving fluid is a gas, and the driving means alternates pressure between comparatively high and low pressures, the high pressure being at or below atmospheric pressure and the low pressure being significantly below atmospheric. The application of the low pressure causes the bladder to expand and application of the high pressure causes the bladder to contract.
The invention also encompasses a method of pumping. A preferred method comprises the steps of (a) providing an extensible bladder having an inlet and an outlet; (b) connecting the inlet and outlet of the bladder to a person""s circulatory system; (c) expanding the interior surface area and volume of the bladder to draw blood into the bladder through the inlet; (d) contracting the interior surface area and volume of the bladder to pump blood out of the outlet of the bladder; and (e) rhythmically repeating steps (c) and (d).
The bladder is preferably made of an elastic material that changes surface area during the pumping cycle. It expands or stretches during the filling phase of the pumping cycle and it returns elastically to its contracted size during the discharge or ejection phase of the pumping cycle. A majority of the interior surface area of the pumping chamber expands and contracts a significant amount (more than a few percent) in each cycle. The change in the area of the bladder surface during the pumping cycle will reduce the incidence and growth of blood clots forming on the surface of the bladder.
In addition, the blood pump of the invention may include variations in the thickness of the bladder and the material comprising the bladder in different areas, segments or portions of the bladder. The thinner areas will stretch more than the thick areas during the filling phase of the pumping cycle. This will draw more blood into the region of the pumping chamber surrounded by the thinner areas of the bladder. Varying the material in different areas of the bladder can also change the amount that various portions of the bladder stretch during the filling portion of the pumping cycle, and in addition, can change the speed at which different areas return to their neutral positions during the ejection part of the pumping cycle. Blood in some areas of the pumping chamber can thus be ejected earlier than blood in other areas. Accordingly, the characteristics of flow into the pumping chamber, through the pumping chamber, and out of the pumping chamber can be controlled and directed. Areas of stagnation can be minimized, further decreasing the likelihood of blood clot formation. Turbulence can also be minimized improving the efficiency of the pump and mitigating damage to blood cells.
In addition to varying the thickness and the material of the bladder, struts of varying elasticity can be molded into the bladder. These struts will bridge from one side to another side of the bladder and aid in maintaining the geometrical shape of the bladder. These struts may also be stretched during the filling portion of the pumping cycle and will provide additional force for ejection during the ejection part of the pumping cycle.
The filling phase of the pumping cycle is advantageously driven pneumatically or by other means for exerting below atmospheric pressure in the space between the housing and the bladder. During the filling phase, blood will be drawn into the pumping chamber and elastic energy will be stored in the bladder. The ejection phase of the cycle will then occur when the negative pressure is released and the bladder returns elastically to its neutral position. Although some positive pressure may be used on the bladder during the ejection phase of the pumping cycle, preferably and ideally there will be no positive pressure exerted on the bladder and all the force for ejection will come from the elastic recoil of the bladder. In this situation, when the power for ejection comes entirely from the elasticity of the bladder and no positive fluid pressure is exerted on the bladder, should a break or tear occur in the bladder, there is very little chance that any significant amount of the driving fluid would enter the circulatory system as there is no positive pressure to drive it through the tear or break in the bladder.
For the foregoing reasons, the blood pump of the invention decreases the likelihood of blood clots forming, improves the pumping characteristics of the device, and decreases or eliminates the chance of foreign fluids passing into the blood stream should a tear or break occur in the bladder. Although the pump of the invention was initially conceived for pumping blood, it also will find utility for pumping fluids in industrial and non-medical fields. Other attributes and benefits of the present invention will become apparent from the following detailed specification when read in conjunction with the accompanying drawings.